A sad fact of life lately at the Museum of Modern Art is that when it comes to group shows of contemporary painting from the collection, the bar has been set pretty low. "What Is Painting?," the crowded 50-artist, 50-work exhibition that opened quietly in July (and closes Sept. 17, 2007), has been a welcome exception, especially if graded on the MoMA curve. It is better than all of the second-floor installations of the permanent collection that have taken place since MoMA’s reopening three years ago, because it defines a polemic about a single medium and then cogently explores it.